In MRI, a uniform strong magnetic field is generated by a magnet, where precession frequencies of protons in an organism are determined by the field strength and are consistent with each other. If a linear gradient magnetic field is added into the main magnetic field, the precession frequencies of proton groups in various parts of a subject being detected may be distinguished from each other due to different magnetic induction intensities, and thus MRI may be performed on a certain part of the subject being detected. A radio frequency (RF) pulse may excite the protons to generate resonance. When a precession frequency of a proton is the same as a center frequency of the RF pulse, energy exchange can occur, and a proton with a low energy obtains energy to enter into a high energy state, i.e., generating nuclear magnetic resonance (NMR). The center frequency of the RF pulse is a very critical parameter in MRI. If the center frequency is not accurate, it will directly result in misalignment of the obtained MR image.
Usually, the center frequency is calibrated by pre-scanning During the pre-scanning, a system may be adjusted using a specially-made MR sample model to determine an optimal resonance frequency (center frequency). It is very accurate to calibrate the center frequency by pre-scanning. However, the method of pre-scanning requires a carrying bed be located at an initial position and kept still. For a carrying bed that is constantly moving, it is very difficult to calibrate the center frequency by the method of pre-scanning.
Therefore, a method for calibrating the center frequency of MR is desired.